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1. Temperature induced optical scatter changes in titania-germania coatings

   https://doi.org/10.1088/1742-6596/3177/1/012107  

   Kapasi, D P; Counihan, T; Smith, JR; Tait, S; Billingsley, G (February 2026, Journal of Physics: Conference Series)

   Abstract Titania doped with tantala is the high index material (high n) for the optical coatings used in LIGO and Virgo and its thermal noise limits LIGO/Virgo observations of astrophysical sources. In this paper, we study temperature induced changes to optical scatter of a multilayer highly reflective coating comprised of silica (low n) and titania doped with germania (high n) as a potential candidate to reduce coating thermal noise in ground-based observatories operating at room temperature. We observe that the scatter measured at 8° in a small region is low, with a median starting Bidirectional Reflectance Distribution Function (BRDF) of 1.1 × 10⁻⁷str⁻¹increasing to 1.2 × 10⁻⁶str⁻¹through annealing. The results presented here show the potential of adopting titania doped with germania coatings for future upgrades to LIGO and Virgo and as a pathfinder coating for Cosmic Explorer, a next-generation detector. 

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2. Enhanced medium-range order in vapor-deposited germania glasses at elevated temperatures

   https://doi.org/10.1126/sciadv.abh1117  

   Yang, Le; Vajente, Gabriele; Fazio, Mariana; Ananyeva, Alena; Billingsley, GariLynn; Markosyan, Ashot; Bassiri, Riccardo; Prasai, Kiran; Fejer, Martin M.; Chicoine, Martin; et al (September 2021, Science Advances)

   Glasses are nonequilibrium solids with properties highly dependent on their method of preparation. In vapor-deposited molecular glasses, structural organization could be readily tuned with deposition rate and substrate temperature. Here, we show that the atomic arrangement of strong network-forming GeO 2 glass is modified at medium range (<2 nm) through vapor deposition at elevated temperatures. Raman spectral signatures distinctively show that the population of six-membered GeO 4 rings increases at elevated substrate temperatures. Deposition near the glass transition temperature is more efficient than postgrowth annealing in modifying atomic structure at medium range. The enhanced medium-range organization correlates with reduction of the room temperature internal friction. Identifying the microscopic origin of room temperature internal friction in amorphous oxides is paramount to design the next-generation interference coatings for mirrors of the end test masses of gravitational wave interferometers, in which the room temperature internal friction is a main source of noise limiting their sensitivity. 

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3. Ar transport and blister growth kinetics in titania-doped germania-based optical coatings

   https://doi.org/10.1088/1361-6382/ad3ffb  

   Lalande, Émile; Davenport, Aaron; Marchand, Lory; Markosyan, Ashot; Martinez, Daniel; Paolone, Annalisa; Rezac, Michael; Bazzan, Marco; Chicoine, Martin; Colaux, Julien L; et al (May 2024, Classical and Quantum Gravity)

   Blistering is a phenomenon sometimes observed in sputtered-deposited thin films but seldom investigated in detail. Here, we consider the case of titania-doped germania (TGO)/silica multilayers deposited by ion beam sputtering. TGO is a candidate as high refractive index material in the Bragg mirrors for the next iteration of gravitational waves detectors. It needs to be annealed at 600^∘C for 100 h in order to reach the desired relaxation state. However under some growth conditions, in 52-layer TGO/silica stacks, blistering occurs upon annealing at a temperature near 500^∘C, which corresponds to the temperature where Ar desorbs from TGO. In order to better understand the blistering phenomenon, we measure the Ar transport in single layers of TGO and silica. In the case of $<$ 1µm-thick TGO layers, the Ar desorption is mainly limited by detrapping. The transport model also correctly predicts the evolution of the total amount of Ar in a 8.5µm stack of TGO and silica layers annealed at 450^∘C, but in that case, the process is mainly limited by diffusion. Since Ar diffusion is an order of magnitude slower in TGO compared to silica, we observe a correspondingly strong accumulation of Ar in TGO. The Ar transport model is used to explain some regimes of the blisters growth, and we find indications that Ar accumulation is a driver for their growth in general, but the blisters nucleation remains a complex phenomenon influenced by several other factors including stress, substrate roughness, and impurities. 

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4. Ar transport and blister growth kinetics in titania-doped germania-based optical coatings

   Lalande, Emile; Davenport, Aaron; Marchand, Lory; Markosyan, Ashot; Martinez, Daniel; Paolone, Annalisa; Rezac, Michael; Bazzan, Marco; Chicoine, Martin; Colaux, Julien; et al (March 2024, Classical and quantum gravity)
5. Annealing‐Induced Changes in the Atomic Structure of Amorphous Silica, Germania, and Tantala Using Accelerated Molecular Dynamics

   https://doi.org/10.1002/pssb.202000519  

   Prasai, Kiran; Bassiri, Riccardo; Cheng, Hai-Ping; Fejer, Martin M. (May 2021, physica status solidi (b))

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